This invention relates to silicon oxide containing active silicon and a method for evaluating the same.
Silicon oxide is well known in the art and generally used as a raw material for the manufacture of ceramics, a raw material for the manufacture of organosilicon compounds, and a negative electrode active material in lithium ion secondary batteries. By taking advantage of its chemical activity, silicon oxide is used in the synthesis of industrially useful alkylhalosilanes (Gary N. Bokerman et al., U.S. Pat. No. 5,051,247) and the direct synthesis of siloxanes (Peter L. Timms and William N. Rowlands, EPA 0406000A2). It is reported in E. Fuglein and U. Schubert, Chem. Mater., 1999, 11, 865-866 that silicon oxide is reacted with magnesium at low temperature to produce magnesium silicide. JP-A 9-7638 discloses that using SiOx as a negative electrode active material in lithium ion secondary batteries facilitates occlusion and release of lithium ions. The use of silicon oxide as the lithium ion secondary battery negative electrode active material is expected to expand in the near future.
A need exists for silicon oxide finding effective use in these applications.
An object of the invention is to provide silicon oxide containing active silicon which finds effective use in the above-mentioned applications and a method for evaluating the same.
We produced samples of active silicon-containing silicon oxide under different conditions and analyzed them for evaluation. We have found from the results of measurement by solid state NMR (29Si DD/MAS) that active silicon-containing silicon oxide, in which two peaks of specific chemical shifts appear definitely separated and the area ratio of these peaks is within a specific range, achieves the above object because it is more reactive with other elements when used in the above-mentioned applications. Whether or not a silicon oxide sample is appropriate can be judged by this method.
In one aspect, the invention provides silicon oxide containing active silicon represented by the general formula: SiOx wherein x is from 0.8 to 1.9. On analysis by solid state NMR (29Si DD/MAS) with a sufficient relaxation time set, the silicon oxide exhibits a spectrum having two separate peaks, a broad peak (A1) centered at xe2x88x9270 ppm and a broad peak (A2) centered at xe2x88x92110 ppm. The area ratio A1/A2 of these peaks is in the range of 0.1xe2x89xa6A1/A2xe2x89xa61.0.
In another aspect, the invention provides a method for evaluating silicon oxide, comprising the steps of analyzing silicon oxide by solid state NMR (29Si DD/MAS) with a sufficient relaxation time set; and judging the silicon oxide to be silicon oxide containing active silicon when the measured spectrum has a broad peak (A1) centered at xe2x88x9270 ppm and a broad peak (A2) centered at xe2x88x92110 ppm, the area ratio A1/A2 being in the range of 0.1xe2x89xa6A1/A2xe2x89xa61.0.